Optical amplifiers have gained acceptance for use in fiber optic transmission in telecommunication and cable television systems. The most common form of amplifier is the erbium-doped fiber amplifier, which consists of a singlemode optical fiber with a core doped with erbium atoms. The fiber is generally configured so that a pump beam at a wavelength near 980 or 1480 nanometers propagates in the same doped fiber as a signal beam at a wavelength near 1550 nanometers. The erbium atoms absorb energy from the pump beam and end up in an excited state that provides optical gain at wavelengths near 1550 nanometers. Although these devices are widely used, they only work efficiently with high-brightness singlemode pump sources at wavelengths that coincide with the relatively narrow and weak erbium pump absorption bands. In practical systems, the power achievable in such devices is limited by the pump power obtainable from a laser diode.
A common way of increasing the output power of a laser diode is to increase the emitting area. This makes it possible to increase the power without increasing the power density at the output facet of the device. Unfortunately, the transverse mode structure of the resulting broad-area laser is multimode, and the laser output is no longer sufficiently coherent to be coupled into a single-mode fiber. Such a diode can, however, be coupled into a multimode fiber to provide an essentially incoherent, high power multimode source.
A multimode pump source can be used to excite a singlemode amplifier if the device is built in the form of a cladding-pumped amplifier. In this configuration, a strongly absorbing singlemode core is embedded in a multimode waveguide, typically a glass- or polymer-clad, step-index, multimode optical fiber. The multimode waveguide is configured so that its modes all have overlap with the absorbing core. When pump light is launched into the multimode waveguide, it is attenuated by absorption in the singlemode core, creating a population of excited atoms concentrated in the singlemode core of the fiber. Signal light propagating in the fundamental mode of the singlemode core extracts the energy from the excited atoms, until a large fraction of the energy absorbed from the multimode pump is transferred to the singlemode signal. Such devices were first described in U.S. Pat. No. 3,808,549, to Maurer, issued Apr. 30, 1974, and developed in further detail in U.S. Pat. No. 4,815,079 to Snitzer et al., issued Mar. 21, 1989. The '549 patent describes a fiber where the singlemode and multimode waveguides form a round and concentric double-clad fiber. The limitations of such a design are described in the '079 patent, where it is described how skew rays in the round waveguide never intersect the absorbing core. The '079 patent, describes other, lower-symmetry multimode waveguide geometries that induce all rays to intersect with the core. More recently, the application "Optical Fiber With Irregularities At Cladding Boundaries," that is incorporated by reference herein, describes how small geometric perturbations of a round, concentric double-clad fiber can be used to produce modes that overlap the absorbing core, to give efficient absorption in a fiber that is mechanically very close to a round, concentric fiber. This application also emphasizes the advantages of using a fiber with an outer cladding layer made from a fluorosilicate glass, as opposed to the polymeric outer cladding used in most of the embodiments in the '079 patent.
A suitable strongly-absorbing core material is the ytterbium-erbium ("Yb, Er") co-doped material described in U.S. Pat. No. 5,225,925, to Grubb et al., issued Jul. 6, 1993. In the optimized fiber disclosed in the '925 patent, energy absorbed by the ytterbium ions is efficiently transferred to the erbium ions. This gives a core material with a much stronger, broader absorption than can be obtained in a singly-doped erbium fiber. The narrow erbium absorption peak near 975 nm is replaced by a ytterbium absorption that extends from at least 900 nm to 1000 nm. Incorporating this core material into a cladding-pumped device and pumping with a multimode source at a nominal wavelength of 950 nm will produce optical gain near 1550 nanometers. A cladding-pumped Yb, Er doped fiber amplifier ("YEDFA") made from this material was demonstrated by Minelly et al. (IEEE Photonics Technology Letters, 5(3), 301-303, 1993), using bulk optics to couple the output of a laser diode array to the double clad fiber. Geometries using fused or reflective couplers similar to those used for conventional single-mode amplifiers have also been used.
Double-clad fibers are not truly single mode fibers, and this has important consequences for the noise properties of amplifiers made from such fibers. In general, there are two types of modes in these fibers: a fundamental mode associated with the "singlemode" core, and a large number of higher-order modes guided by the outer boundary of the multimode waveguide. Mixing between these two types of modes is important because signal light launched into the cladding modes at one point will couple back into the core at a later point with a delay introduced by the differing propagation constants. Interference between the delayed signals gives amplitude fluctuations characterized as multi-path, interferometric noise on the output of the signal beam.
Under ordinary circumstances, there is very little coupling between the two groups of modes, not only because the modes are orthogonal, but also because the propagation constant of the fundamental mode is significantly different from those of the cladding modes. Small effects such as Rayleigh scattering do transfer a small amount of power continuously between modes over the length of the fiber and, in the case of a pumped fiber, spontaneous emission generates optical power in both the core and the cladding. However, these effects are small compared to what happens at the fiber end interfaces where the signal is coupled in and out of the doubleclad fiber through a free-space coupling or a fusion splice. Because of imperfections in the coupling conditions due, for example, to small core misalignments and differences in core compositions, the two groups of modes are much more strongly coupled at these interfaces. Launching a beam from one singlemode fiber into another singlemode fiber, whether by a fusion splice or by some form of free-space coupling, is never 100% efficient, with the "lost" light being coupled to higher order modes. In conventional singlemode fibers these modes are attenuated by a high index polymer applied to the outside diameter of the fiber. This coating prevents total internal reflection at the fiber outer diameter, and the optical power in the higher-order modes is transmitted into the polymer layer where it is dissipated by scattering. This power is not lost in a doubleclad fiber, because the outer cladding layer guides both the pump light and the undesired signal light.
While the dominant source of signal power in higher order modes is associated with launch imperfections that occur at the input to the fiber, Rayleigh scattering and spontaneous emission contribute additional signal throughout the entire length of the fiber. An important difference is that the launch imperfections and Rayleigh scattering produce delayed versions of the original signal that can lead to multi-path (coherent) noise, while the spontaneous emission leads to incoherent noise. Light from all three sources can be amplified as it passes through a fiber with gain, so that significant power may be present in the higher order modes at the output of the fiber. Power scattered by launch imperfection generally constitutes the largest contribution.
At the output of the fiber, a conventional singlemode fiber gives a simple, nearly-Gaussian beam with no significant spatial structure. The output of the doubleclad fiber is more complicated. Although the majority of the power is in the singlemode core, there is enough power in higher order modes to produce interference that results in clearly visible fringes on the output beam. This can be readily observed by launching visible light into a doubleclad fiber, or by viewing an infrared output with a suitable viewer. Although the divergence of the beam is similar to that from a singlemode fiber, the detailed structure of the output varies greatly as the fiber is twisted or bent, as would be expected for multimode fiber. Splicing this fiber to another singlemode fiber produces an output that is spatially singlemode, but with the visible spatial structure transformed into intensity modulations that can be readily observed with a photodiode. The relationship of these modulations to the higher order modes is evident because twisting or bending the fiber results in significant amplitude modulation of the singlemode output signal.
It is useful to note that the degree of mixing at a splice is determined by the spatial overlap between each higher order mode and the singlemode core. The higher order modes of a round, concentric doubleclad fiber have very little overlap with the singlemode core. Thus, when singlemode power is launched into a round, concentric doubleclad fiber, the spatial effects noted above are relatively weak, and the noise generated on splicing this fiber to a singlemode fiber is small. The effects are much stronger if a less symmetric shape is used so that nearly all modes overlap the core. Using such less symmetric shapes is highly desirable in cladding pumped amplifiers because it allows efficient pumping; unfortunately it also causes the undesired coupling of the signals between the fundamental and higher order modes at the splices.
A problem with noise arises from cladding modes at the signal wavelength. Power coupled into higher-order modes mixes with the fundamental mode to produce noise. Accordingly, a need exists for systems and methods for an optical fiber for use with an optical amplifier that suppresses cladding modes at the signal wavelength without perturbing the pump wavelength.